Membrane-based separation technologies have the potential to separate CO2 at reduced costs under certain conditions, relative to other technologies such as solvent-based absorption methods. The performance of membrane-based separation technologies is sensitive to the CO2 concentration in the feed gas, this is because the CO2 concentration directly impacts the driving force for separation.
One use for CO2 separation technologies is in the reduction of CO2 emissions through the capture of CO2 from power plant flue gases. The concentration of CO2 in the flue gas of a power plant can vary over time, depending on various factors such as the operating load of the power plant. For example, when a coal-fired power plant is operated at full capacity, the concentration of CO2 is about 13-14%. When the power plant is operated a lower load, the CO2 concentration in the flue gas can drop below 12%.
For membrane systems, the reduced pressure driving force could increase the cost of CO2 removal, reduce the operating thermal efficiency of the power plant through additional auxiliary loads due to compression or pumping, and, in some cases, compromise the ability to achieve the target levels of CO2 removal. Therefore, dynamic changes in the CO2 concentration, such as those encountered in response to load following can make steady state operation of membrane-based separation systems more difficult.
Efforts to optimize the performance of membrane-based systems for CO2 capture have focused on three general approaches for maximizing the driving force for separation: (1) Increasing the overall pressure of the feed through compression; (2) Decreasing the pressure on the permeate side using vacuum: and (3) Increasing the concentration in the flue gas feed stream using recycle loops. In the first approach, the low concentration of CO2 in flue gas streams typically means that significant compression is required to meaningfully improve performance. This results in high parasitic auxiliary loads and reduced power plant efficiency. The second approach has been used in some designs. However, since the permeate pressure can only be reduced about 1 atmosphere, the improvement in the driving force for separation is limited. The third approach involves recycling part of the retentate to increase the concentration of CO2 in the feed. An example of this is exhaust gas recirculation (EGR), which involves recirculation of the retentate to the combustor, with the effect of increasing the CO2 concentration.